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Here, we report on the hybrid hole transport materials 4,4′-bis-(carbazole-9-yl)biphenyl (CBP) or poly-N-vinylcarbazole (PVK) doped into poly(4-butyl-phenyl-diphenyl-amine) (Poly-TPD) as the hybrid hole transport layer (HTL) to tailor the energy band alignment between hole injection layer (HIL) and quantum dot (QD) light emitting layer in order to realize efficient quantum dot light emitting diodes (QLEDs) in all solution-processed fabrication. Compared to the pristine Poly-TPD based device, it is found that the electroluminescence (EL) performance of QLEDs can be significantly improved by 1.5 fold via addition of CBP into Poly-TPD, which can be attributed to the lowered highest occupied molecular orbital (HOMO) level of Poly-TPD to reduce the energy barrier between HTL and valance band (VB) of QDs. Thus, after doping small molecules into polymer under optimized proportion (Poly-TPD:CBP = 2:1 by weight), the hole transport rate can be balanced, facilitating the carrier injection from HTL to QDs and enhancing the efficiency of QLEDs. As a result, a maximum luminance, a maximum current efficiency and a maximum power efficiency of 7600 cd/m2, 5.41 cd/A and 4.25 lm/W can be obtained based on this variety of hybrid HTL employed QLEDs.
© 2016 Optical Society of America
1. Introduction
Quantum dot light-emitting diodes (QLEDs) have attracted much attention in the past few years since they possess unique properties of tunable emissions by controlling the size of QDs, highly saturated emission, narrow emission with small full width at half maxima (FWHM), solution process, and compatibility with flexible substrates [1–6]. Ever since the first demonstration of QLEDs [7], great efforts have been made to achieve better device performance, in terms of suitable charge transport/injection materials and optimization of device structures [5, 6].
However, the imbalanced charge transport due to excess electrons, either charging the QDs and consequently enhancing the nonradiative Auger recombination, or transporting to the counter electrode without recombination, which are considered as serious bottleneck for the improvement of light emission efficiency [8]. In the previous report, Dai et al. reported the high-performance QLEDs by inserting an insulating layer between the QD layer and the metal oxide as electron transport layer (ETL) to reduce the electron flow and optimize charge balance in the device and preserve the superior emissive properties of the QDs [9].
In the conventional QLEDs, the most commonly used hole transport layer (HTL) is organic poly[bis(4-butypheny)-bis(phenyl)benzidine] (Poly-TPD), which shows good hole transport capability with the highest occupied molecular orbital (HOMO) level of −5.2 eV [10] and high-quality film surface. Generally, the conduction band (CB) (~−4 eV) of QDs is similar to the CB of metal oxide ETL, such as ZnO (~−4.2 eV) [3], TiO2 (~−4.3 eV) [11], but the valence band (VB) (~−6 to −7 eV) is much lower than the HOMO of typical conjugated organic molecules or polymers, such as, Poly-TPD [2]. Thus, the energy barrier between Poly-TPD layer and QDs is more than 1 eV, which is much larger than that between ETL and the QDs. In other words, the energy band position of the QD favors more electrons to be injected into QDs than holes. It has been reported that the performance improvement of QLEDs critically depends on the exploitation of p-type layers to boost the hole injection [12, 13]. Therefore, it is highly desired to select suitable HTL materials to balance the electrons and holes transport capability, improving the performance of QLEDs. Recently, various methods have been proposed to form suitable HTLs in order to overcome these crucial issues. For example, phase separation was a convenient methodology to easily obtain the individual HTL and QD layer from the mixed solution of HTL and QD [14]. Metal oxides, such as MoO3, NiO, or WO3, have been utilized as HTLs by vacuum evaporation and magnetron sputtering methods due to their merits of stability and reproducibility compared with organic HTLs [15, 16]. In addition, by introducing double hole transport layers (HTLs), the performance enhancement is attributed to the stepwise HTL structure, which can decrease the hole-injection barrier from HTL to QD emitting layer and reduce the turn-on voltage of QLEDs [16].
Herein, we propose hybrid organic HTLs by doping 4,4′-bis-(carbazole-9-yl)biphenyl (CBP) or poly-N-vinylcarbazole (PVK) into Poly-TPD as HTLs and fabricate ZnCdSeS based QLED device by solution process. Compared with vacuum evaporation and magnetron sputtering, solution process offers the potential for the commercialization of QLEDs due to the advantages of roll-to-roll technique, large area,and low cost production.
2. Experimental section
2.1 Chemicals
All reagents used in this study were purchased at anlytical reagent grade and used for synthesis without further purification. Cadmium oxide (CdO, 99.9%), zinc acetate [Zn(acet)2, 99.99%], selenium (Se, 99.9%), sulfur (S, 99.9%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), trioctylphosphine (TOP, 90%), dimethyl sulfoxide (DMSO, HPLC grade, 99.7%), tetramethylammonium hydroxide (TMAH, 97%) were purchased from Sigma-Aldrich. Poly(4-butyl-phenyl-diphenyl-amine) (Poly-TPD) were purchased from Shanghai Han Feng Chemical Co.,Ltd. 4,4′-bis-(carbazole-9-yl)biphenyl (CBP, 99.9%), poly-N-vinylcarbazole (PVK) were purchased from Xi’an Polymer Light Technology Corp. Acetone (99.5%), toluene (99.5%), butanol (99.5%) and ethanol (99.5%) were obtained from Nanjing Zhongdong chemical Co.,Ltd.
2.2 QD synthesis
Green emitting ZnCdSeS QDs were synthesized according to a modified method reported previously [17]. In this study, 0.4 mmol of CdO, 4 mmol of zinc acetate, 4 mmol of oleic acid (OA), and 20 mL of 1-octadecene were mixed in a round flask of 100 mL. The mixture was heated to 150 °C for 30 min degassed under ∼10 pa pressure, filled with high-purity N2 flowing, and further heated to 300 °C to form a clear solution of Cd(OA)2 and Zn(OA)2. At this temperature, a stock solution containing 3 mL of trioctylphosphine, 0.4 mmol of Se, and 4 mmol of S was quickly injected into the reaction flask. After the injection, the reaction temperature was maintained for 10 min to promote the growth of QDs. The reaction was subsequently cooled down to room temperature to stop further growth. The QDs were washed with acetone for three times, and finally dispersed in toluene at a concentration of 10 mg/mL.
2.3 ZnO synthesis
The ZnO NPs used in this study were synthesized through a sol−gel method. A solution of 0.1 M zinc acetate in dimethyl sulfoxide (DMSO) and 10 mL of 0.5 M tetramethylammonium hydroxide (TMAH) in ethanol were mixed and stirred for 1 h in ambient atmosphere.The prepared product was collected by centrifugation and then washed twice with methanol. The transparent precipitate was re-dispersed in ethanol to form a ZnO NPs solution with concentration 30 mg/mL.
2.4 Device fabrication
The fabrication of QLEDs was completed in a nitrogen-filled glovebox, which normally has oxygen and moisture levels below 0.1 ppm located in a class 10000 clean room. The fabrication procedure was described as follows: The QLEDs were fabricated on glass covered with indium tin oxide (ITO). The substrates were first cleaned with de-ionized water, acetone and iso-propanol, consecutively, for 15 min each, and then treated with ultraviolet light generated ozone for half an hour. Poly(ethylenedioxythiophene):poly styrenesulphonate (PEDOT:PSS) solutions were spin-coated onto the ITO substrates at 5000 rpm for 30 s and baked at 120 °C for 20 min under ambient conditions. The PEDOT:PSS-coated substrates were transferred into a nitrogen-filled glove box (O2< 0.1 ppm, H2O < 0.1 ppm) for spin-coating of the sequential layers. The poly(N,N9-bis-(4-butylphenyl)-N,N9-bis(phenyl)-benzidine) (Poly-TPD) used as the hole transport layer (1 wt% in chlorobenzene) was spin-coated at 2500 rpm for 30 s, followed by baking at 110 °C for 30 mins. Then QDs (10 mg/ml in toluene) were layered at 800 rpm for 30s, followed by baking at 120°C for 30 mins. After that, the ZnO NPs (30 mg/ml in butanol) were spin-coated at 4000 rpm for 30 s and baked at 120 °C. Finally, the top Al cathode was deposited in a custom high-vacuum deposition chamber (background pressure, 6 × 10−4 torr) with an active device area of 120 mm2.
2.5 Characterization
The structural analysis of the samples was carried out using a Cs-corrected high-resolution transmission electron microscope (HRTEM, Tecnai G20). The current−voltage (I−V) characteristics were measured with a Keithley-2400 source-meter unit, and the absorption and photoluminescence (PL) spectra measured using the U-4100 UV−visible and NIR-300 spectrophotometer, respectively. The morphology of the sample was displayed by Atomic Force Microscope (a Park XE-120 atomic force microscope). EL spectra and luminance−current density−voltage characteristics of QLEDs were obtained with a Konica-Minolta CS-2000 spectroradiometer coupled with a Keithley 2400 voltage and current source under ambient conditions.
3. Results and discussion
The absorption and photoluminescence (PL) spectra of ZnCdSeS QDs used in this study are shown in Figs. 1(a). The absorption spectrum clearly displays the first excitonic transition peak located at 530 nm. The PL spectrum shows a Gaussian-shaped peak located at 550 nm, with a narrow FWHM of ~30 nm. The high resolution transmission electron microscope (HRTEM) image, as shown in Fig. 1(b), indicates that ZnCdSeS QDs without core-shell structure were uniformly dispersed in toluene with an average diameter of ~7 nm. The quantum yield (QY) of ZnCdSeS QDs can be calculated as 70%, using Rhodamine 6G as reference.
Fig. 1 (a) Absorption and PL spectra of QDs; (b) TEM image of QDs; (c) the structure schematic diagram and; (d) energy level diagram for the various layers of the QLEDs.
The schematic of our device structure and corresponding energy level diagram are shown in Figs. 1(c) and 1(d), respectively. The device has a structure of ITO/PEDOT:PSS (40 nm)/HTL (30 nm)/QDs (20 nm)/ZnO (30 nm)/Al (100 nm). Electrons are injected from the Al as cathode, and meanwhile holes are injected from the ITO as anode. The excitons recombine in the QD layer sandwiched between HTL and ETL and emits green light. From the schematic energy level diagram in Fig. 1(d), the electrons can be easily injected from the Al to the QD layer and holes injected from the ITO can be confined within the QD layer due to the electron affinity of 4.5 eV and ionization potential of 7.3 eV for ZnO. Thus, the ZnO nanoparticle (NP) layer not only provides efficient electron injection from the Al cathode into ZnCdSeS QDs, but also helps to block holes within the QD layer due to the valence band offset at the QD/ZnO NPs interface, leading to an improved charge recombination. However, the case is different for the hole transport considering the larger energy barrier between the HTL and QDs, which is attributed to the relatively shallow HOMO energy level of sole HTL. An effective method to solve this problem is to reduce the HOMO energy level of HTL bydoping HTL with organic materials lower HOMO energy level to facilitate the hole transport. In order to exploit the doping effect, the various HTL materials will be firstly explored.
Various polymer materials and small molecules have been employed as hole transport materials for QLEDs, including Poly-TPD, CBP, and PVK. It is well known that the film uniformity has significant effect on the carrier transport and thus the performance of QLED devices due to the layer-by-layer solution process [11]. The surface roughness of different HTLs coated on PEDOT:PSS are displayed in Figs. 2(a)-2(e). It can be observed that the roughness of Poly-TPD is the smallest (0.64 nm) while that of CBP is the largest (2.08 nm), indicating that the CBP layer is not smooth as Poly-TPD or PVK. This is ascribed to the low packing densities of the small molecule thin films [18]. In addition, CBP has rather low glass transition temperatures (Tg of ~62°C) [19]. Consequently, CBP is easily crystallized, resulting in poor roughness of the film. On the other hand, there is no obvious difference on the roughness among the pristine Poly-TPD layer, Poly-TPD:PVK layer and Poly-TPD:CBP layer. Based on the results in Figs. 2(a)-2(e), it can be speculated that the QLED device utilizing Poly-TPD as HTL material works better than the other ones.
Fig. 2 AFM images of the different HTL spin coated on the PEDOT:PSS (a)Poly-TPD, (b) PVK, (c) CBP,(d) Poly-TPD:PVK, (e) Poly-TPD: CBP. The roughness of root mean square is indicated in the figure.
The EL characteristics of QLEDs with various HTL materials are illustrated in Figs. 3(a)-3(d). The performance summary of QLEDs with various HTL materials is tabulated in Table 1. It is found that the current density of QLED with only Poly-TPD is the lowest in Ohmic region, compared with other devices with sole HTL, which indicates the smallest leakage current [20]. That is in accordance with the result that the of PEDOT:PSS/Poly-TPD has the best uniformity as mentioned above. The values of current and power efficiencies for CBP-based QLED devices are much lower than those of polymer materials (Poly-TPD and PVK) based devices, shown as Figs. 3(b) and 3(c). These results are attributed to the low packing densities of small molecule thin films. Therefore, small molecules themselves alone as the HTL are not suitable for high-performance QLED application in spite of their high hole mobility and low HOMO levels. In addition, the Poly-TPD based QLED shows higher current efficiency and power efficiency than the PVK based devices. On one hand, these results can be attributed to the higher hole mobility of Poly-TPD (1.0 × 10−4 cm2/V·s) than that of PVK (2.5 × 10−6 cm2/V·s) [21], which enhances the hole transport. On the other hand, the better film quality of Poly-TPD contributes to the carrier transport.
Fig. 3 (a) The current density versus voltage of device; (b) current efficiency and (c) power efficiency of device; (d) EL spectra of device at different driving voltage, inset shows the real picture of working device.
Table 1. Summary of device performances for different hole transport materials.
Though our best results originate from the Poly-TPD based devices, its HOMO is limited to −5.2 eV, resulting in the high turn-on voltage and low efficiency of the devices due to the large barrier for hole transport between Poly-TPD and QD layers. To overcome this problem, different HTL materials like PVK or CBP are doped to the Poly-TPD layer to decrease the HOMO of HTL. The weight ratio of Poly-TPD to CBP (or PVK) is 2:1. The decreased turn-on voltage and increased current density indicate that the hole injection is improved by adding PVK or CBP into Poly-TPD, as shown in Fig. 3(a). The current efficiency and power efficiency of QLEDs is significantly enhanced from 3.71 cd/A to 5.41 cd/A and 2.59 lm/W to 4.25 lm/W, respectively, shown in Figs. 3(b) and 3(c), and Table 1. It is remarkably enhanced by utilizing CBP as a dopant. While a slight increase can also be seen for the QLED based on the PVK as a dopant, shown in Table 1. These results can be explained by the lower HOMO level of CBP compared to that of PVK. The energy band offset between Poly-TPD layer and QD layer is about 1.6 eV, leading to the larger barrier for holes transporting from HTL to emitting layer. On the other side, the energy band offset between ZnO layer and QD layer is about 0.2 eV. Electrons can be transferred much easier than holes to the emitting layer, which results in the imbalance of exciton injection, thereby effecting the performance of devices. Both HOMO levels for PVK (−5.8 eV) [21] and CBP (−6.0 eV) [4] are lying between Poly-TPD and QDs, presents as Fig. 1(d). Therefore it could effectively reduce the energy barrier of the hole injection and promote the carrier injection efficiency by doping PVK or CBP [22, 23]. These results indicate that the HOMO energy level plays an important role in the QLED performance, taking the similar roughness of the hybrid HTLs into account. In addition, the higher hole mobility of CBP (1.0 × 10-3 cm2/V·s) [4], compared with PVK (2.5 × 10-6 cm2/V·s) [21] is more efficient to enhance the conductive in the HTL for the device. It is demonstrated that doping CBP is a better choice than PVK for improving the performance of QLEDs.
Electroluminescence (EL) spectra of the Poly-TPD:CBP based device under different bias are shown in Fig. 3(d). The emission peak of the device under different bias is identical, indicating that the recombination zone does not change with driving voltage, remaining in the QD layer. The EL intensity increases greatly along with the increasing driving voltage. Compared with their PL spectrum, the EL emission peak is slightly red-shifted (~10 nm), which is due to the Förster resonant energy transfer (FRET) from smaller (donor) to larger (acceptor) dots within the QDs [3]. Also, the emission is rather pure, i.e. without peaks from neighboring organic layers, demonstrating that the excitons are effectively confined in the QD layer and recombine [23].
In order to investigate the origin of the performance improvement by doping in HTL, the electron and hole transport characteristics are displayed in Fig. 4. In our device, there is a moderate energetic barrier for hole injection owing to the deep valance-band energy level of the quantum dots, indicated as Fig. 1(d). Furthermore, the hole mobility of Poly-TPD (1.0 × 10−4 cm2/V·s) [10] is one order of magnitude lower than the electron mobility of ZnO nanocrystal films (1.0 × 10−3 cm2/V·s) [9]. These factors can lead to excess electron injection into the QD emissive layer. The unbalanced charge injection in the QLED is reflected by significantly higher current density of the electron-only devices (ITO/QDs/ZnO/Al) than those of the hole-only devices (ITO/PEDOT:PSS/HTL/QDs/Al) as displayed in Fig. 4. The unbalanced charge injection in the QLED layer degrades device performance. The hole injection from the HTL to the QD layers can be modulated by doping different HTL materials. It can be found that the current density of hole only devices (HOD) based on Poly-TPD:CBP is obviously higher than that of HOD based on Poly-TPD-Only, indicating the hole transport is enhanced, which is in accordance with the current-voltage characteristic of QLED device shown in Fig. 3(a). Furthermore, the current density of HOD based on Poly-TPD:CBP is closer to the current density of electron only device (EOD) compared with that of HOD based on Poly-TPD:PVK, contributing to the better balance of carrier injection. This is because of the lower HOMO level of CBP (−6.0 eV) compared to that of PVK (−5.2 eV), which facilitates the hole transport. It can be concluded that the performance improvement of QLED based on doping HTL is attributed to the carrier balance due to the lower HOMO level andbetter film quality.
Fig. 4 Current versus voltage characteristics for hole only devices (HODs) and electron only device (EOD).
To test the devices’ operational lifetime, both of their operational lifetimes have been tested in air under a constant current operation. The results from different batches of devices indicate that the Poly-TPD:CBP based devices show much longer lifetime (~80%) than the pristine Poly-TPD based device. Lifetime for these devices for these devices is shown Fig. 5. T50 (defined as the time for the luminance reducing to L0/2) is 45 h and 33 h respectively for the devices based on Poly-TPD:CBP and pristine Poly-TPD. Using the relation L0n × T50 = constant (1.5 ≤ n ≤ 2), and assuming the acceleration factor of n = 1.5 [9], we obtain a T50 of 694 h for the QLEDs based on Poly-TPD:CBP at L0 = 100 cd/m2, which is longer than that based on pristine Poly-TPD of 370 h. It can be concluded that the lifetime of the QLEDs based on Poly-TPD:CBP can be longer compared with that based on pristine Poly-TPD, due to the balance of carrier injection.
4. Conclusion
We investigated the effects of different hole transport materialson the performance of QLEDs. Poly-TPD was found to be a good polymer material for the HTL because of its high hole mobility and high film quality. The HOMO of HTLs was considerably reduced by doping with CBP, closer to the VB of QDs, which contributes to the hole transport. The QLED performance could be largely improved by utilizing the doping HTL because it was in favor of the balance of carrier injection. Thus, we successfully demonstrated a QLED device with a turn on voltage of 2.6 V, a peak current efficiency and power efficiency of 5.41 cd/A and 4.25 lm/W respectively. This strategy provides a new path for tailoring the energy band alignment in QLED with doping engineering, which offers potential for high throughput and practical use.
Funding
National Natural Science Foundation Project (51120125001, 61271053, 61306140, 61405033, 91333118, 61372030, 61307077 and 61674029), Natural Science Foundation Project of Jiangsu Province (BK20141390, BK20130629, and BK20130618), Youth Innovation Promotion Association CAS (No.2013206).
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